[{SiNDipp}MgNa]2: A Potent Molecular Reducing Agent

The bimetallic species, [{SiNDipp}MgNa]2 [{SiNDipp} = {CH2SiMe2N(Dipp)}2; (Dipp = 2,6-i-Pr2C6H3)], is shown to be a potent reducing agent, able to effect one- or two-electron reduction of either dioxygen, TEMPO, anthracene, benzophenone, or diphenylacetylene. In most cases, the bimetallic reaction products imply that the dissimilar alkaline metal centers react with a level of cooperativity. EPR analysis of the benzophenone-derived reaction and the concurrent isolation of [{SiNDipp}Mg(OCPh2)2], however, illustrate that treatment with such reducible, but O-basic, species can also result in reactivity in which the metals provide independent reaction products. The notable E-stereochemistry of the diphenylacetylene reduction product prompted a computational investigation of the PhC≡CPh addition. This analysis invokes a series of elementary steps that necessitate ring-opening via Mg+ → Na+ amido group migration of the SiNDipp ligand, providing insight into the previously observed lability of the bidentate dianion and its consequent proclivity toward macrocyclization.

Agilent ProPulse spectrometer ( 1 H at 500 MHz, 13 C at 126 MHz).The spectra are referenced relative to residual protio solvent resonances.Elemental analyses were performed at Elemental Microanalysis Ltd., Okehampton, Devon, UK.Solvents were dried by passage through a commercially available solvent purification system and stored under argon in ampoules over 4 Å molecular sieves.C6D6 was purchased from Sigma-Aldrich, dried over a potassium mirror before distilling and storage over molecular sieves.[{SiN Dipp }MgNa]2 (4), 1 was prepared according to reported procedures.All other chemicals were purchased from Sigma-Aldrich Merck and used without further purification.
The asymmetric unit comprises half of a dimer plus half of a benzene moiety in 6.The remainder of the bimetallic complex is generated by a crystallographic 2-fold rotation axis, while the solvent is completed by virtue of inversion symmetry intrinsic to the space group.80:20 disorder was taken into account for the benzene content with distance and planarity restraints being employed in the disordered region to assist convergence.
The hydrogen atoms attached to C61 and C62 were located and refined without restraints in the structure of 8.
In 9, the asymmetric unit equates to half of one molecule.The remainder is generated by the crystallographic, 2-fold rotation axis coincident with Mg1.
In addition to one molecule of the bimetallic dimer, the asymmetric unit plays host to a pair of toluene molecules in the structure of 10.The bridging ligand in the dimer was treated for 58:42 disorder, while the toluene based on C85 was modelled to take account of 75:25 disorder.Distance and ADP restraints were employed, on merit, in disordered regions to assist convergence.

Computational Details
DFT calculations were run with Gaussian 16 (C.01). 5The Na, Mg, and Si centres were described with the Stuttgart RECPs and associated basis sets, 6 and the 6-31G** basis set was used for all other atoms (BS1). 7A polarisation function was also added to Si (ζd = 0.284).Initial BP86 optimisations were performed using the 'grid = ultrafine' option, 8 with all stationary points being fully characterised via analytical frequency calculations as minima or transition states (all positive eigenvalues or one imaginary eigenvalue respectively).All energies were recomputed with a larger basis set featuring 6-311++G** basis sets on all atoms (BS2).Corrections for the effect of benzene (ε = 2.2706) solvent were run using the polarisable continuum model and BS1. 9 Single-point dispersion corrections to the BP86 results employed Grimme's D3 parameter set with Becke-Johnson damping as implemented in Gaussian. 10Quantum Theory of Atoms in Molecules (QTAIM, AIMALL program 11 ) analysis was performed on the BP86-optmised geometries PE, using the wavefunction file obtained with Gaussian 16 (C.01)at the BP86/BS2 level.

Breakdown of energy contributions
The following table details the evolution of the relative energies as the successive corrections to the initial SCF energy are included.

Figure S1 :
Figure S1: Connectivity of compound 7 established by a single crystal X-ray diffraction study.

Table S1 :
Crystal Data and Structural Refinement for the Alkaline Earth Compounds 6 and 8 -10.

Table S2 :
Terms used are: Relative energies (kcal mol -1 ) for computed structures.Data in bold are those used in the main text.All energies are quoted relative to I at 0.0 kcal/mol.       /     /    +

Table S3 .
Selected BCP data for 9 Na (BCPs under the threshold of 0.005 are denoted with an asterisk).